knjiga solarna energija dobra

214 BULK CRYSTAL GROWTH AND WAFERING FOR PV
Table 6.1 AM1.5 cell efficiencies for non-textured BSF cells on tri-Si and mono-Si substrates.
Cell area is 103 × 103 mm 2 pseudo square
Material
(non-textured)
Thickness
average
[µm]
Average
efficiency
[%]
V OC
[mV]
I SC
[mA/cm 2 ]
FF
[%]
Rho
[ohm cm]
Tri-Si 140 15.5 615 33.4 75.5 4
Mono-Si 200 15.2 612 32.7 76 1
6.3 BULK MULTICRYSTALLINE SILICON
Multicrystalline silicon besides monocrystalline silicon represents the basis of today’s
photovoltaic technology. Multicrystalline silicon offers advantages over monocrystalline
silicon with respect to manufacturing costs and feedstock tolerance at, however, slightly
reduced efficiencies. Another inherent advantage of multicrystalline silicon is the rectangular
or square wafer shape yielding a better utilisation of the module area in comparison
to the mostly round or pseudosquare monocrystalline wafers. The efficiencies of multicrystalline
silicon solar cells are affected by recombination-active impurity atoms and
extended defects such as grain boundaries and dislocations. A key issue in achieving
high solar cell efficiencies is a perfect temperature profile of both ingot fabrication and
solar cell processing in order to control the number and the electrical activity of extended
defects. Moreover, the implementation of hydrogen-passivation steps in solar cell processing
turned out to be of particular importance for multicrystalline silicon. With the
introduction of modern hydrogen-passivation steps by Si 3 N 4 layer deposition, the efficiencies
of industrial multicrystalline silicon solar cells were boosted to the 14 to 15%
efficiency range and consequently market shares were continuously shifted towards multicrystalline
silicon as the standard material of photovoltaics.
6.3.1 Ingot Fabrication
Two different fabrication technologies for multicrystalline silicon, the Bridgman and the
block-casting process (see illustrations in Figures 6.6 and 6.7) are employed. In both
processes the solidification of high-quality multicrystalline silicon ingots with weights of
250 to 300 kg, dimensions of up to 70 × 70 cm 2 and heights of more than 30 cm have
been successfully realised. While the Bridgman technology is a quite commonly used
technique, the only two companies mainly employing the casting technology are Kyocera
(Japan) and Deutsche Solar GmbH (Germany) [17, 18].
The main difference between both the techniques is that for the melting and crystallisation
process only one crucible (Bridgman) is used, whereas for the crystallisation
process a second crucible (block casting) is used.
In the case of the Bridgman process, a silicon nitride (Si 3 N 4 )-coated quartz crucible
is usually employed for melting of the silicon raw material and subsequent solidification
of the multicrystalline ingot. The Si 3 N 4 coating thereby serves as an anti-sticking layer
preventing the adhesion of the silicon ingot to the quartz crucible walls that owing to
the volume expansion during crystallisation of the silicon material would inevitably lead
to a destruction of both the silicon ingot and the crucible. Concerning the block-casting